4. THE IONISED GAS AND ASSOCIATED OUTFLOWS

As mentioned in Section 1, a signature of
Seyfert nuclei is the
presence of ionised gas. This is believed to be either photoionised
ambient galactic gas (Pedlar, Unger & Dyson 1985; Unger et al. 1987;
Falcke, Wilson & Simpson 1998) or nuclear gas which is ionised and
driven along the radio jets present in AGNs
(e.g. Begelman, Blandford & Rees 1984; Schulz 1988; Colbert et al. 1996;
Colbert et al. 1998). The ionised
material and the observed photons are collimated by the dusty
material (torus) obscuring the continuum source
(Antonucci & Miller 1985; Wilson, Ward & Haniff 1988; Tadhunter &
Tsvetanov 1989; Wilson & Tsvetanov 1994; Baker & Scoville 1998), causing
them to exhibit a sharp linear edge so that the ionised gas is observed
as a bi-conical cone
in the narrow line region(42)
(Storchi-Bergmann, Wilson & Baldwin 1992; Dopita et al. 1998). In this model,
ionised gas which passes through the sublimation radius (where hot
dust radiates in the infra-red) is broken into clouds which are able
to fall back closer to the nucleus, possibly being observed as the
broad line region. Dust and ionised gas which have accreted onto the
AGN are driven back outward in the direction of the jets by radiation
pressure, thus maintaining the direction of the jet flow(43)
(Wilson & Tsvetanov 1994; Capetti et al. 1996),
Table 2(44).

Figure 8. The hydrodynamical model of the
accretion flow and dusty wind as
proposed by Dopita et al. (1998). Courtesy of Mike Dopita.

Table 2. The
Seyfert galaxies known to possess ionisation cones. Sy is the Seyfert
type (obtained from the NASA/IPAC Extragalactic Database),
rion,
CAion are the extent (*for H0 = 75 km
s-1 Mpc-1) and opening angle of the ionisation
cone, respectively, CAradio is the opening angle of the nuclear
radio structure and PA is the
difference in the position angle
of the radio structure and the ionisation cone. This table is adapted from
Wilson & Tsvetanov (1994) but with NGC 2992 added from the results of
Ulvestad & Wilson (1984); Márquez et al. (1998), NGC 4051 from
Christopoulou et al. (1997), NGC 5929 from Su et al. (1996) and Circinus
added from various other works
(see Table 3 for details). **The result for
NGC 1365 is added from the results of
Sandqvist, Jörsäter & Lindblad (1995), who consider the galaxy
as a Seyfert type 1.5
(Véron et al. 1980; Jörsäter, Lindblad & Boksenberg 1984;
Jörsäter & Lindblad 1989).

Table 3. The outflow
properties in Circinus. The
H outflow (Elmouttie et
al. 1998c) is observed towards the NW
only. *Note that Veilleux & Bland-Hawthorn (1997) (also) derive a position
angle of 295° but an
opening angle of ~ 100° for the ionised outflow. In the case of
the radio continuum (Harnett et al. 1990; Elmouttie et al. 1995) and CO
(Curran et al. 1999) observations, outflows are also measured
towards the SE, although, unlike the molecular
outflow, the radio lobe is strongest in the NW.

Radio

H

CO

Position angle

115° and
315°

292±5°*

120° and 300±20°

Inclination angle

-

-90 to 40°

-12°
and 168±10°

Opening angle

15°

66°*

90±5°

Inferred length

±1 kpc

400 to 520 pc

±500 pc

Outflow velocity

-

150 to 200 km s-1

190±10 km s-1

Due to the depletion of the accreting gas, the opening angle of the
cone will increase as the system evolves, thus making Sy1s more
readily observable over a wider range of angles in mature sources
(Dopita 1998). Worth mentioning is that
Pogge (1989) finds that extended ionised gas structures occur more
frequently in Sy2s (as in Table 2), although this
result is from
an admittedly small sample. Ionisation cones are expected to have a
dusty layer form
along their inner edge (Dopita et al. 1998), thus permitting the
presence of molecules along the surface of the outflow
(Curran et al. 1999; Curran, Johansson & Rydbeck 2000).
(45)
In the model of
Dopita et al. (1998), the
ionisation cone and the radio jet (Fig. 8) have
different
origins, i.e. from the dusty torus and from the black hole,
respectively(46)
(Whittle et al. 1988). It should be noted, however,
that the generally small scales and wide opening angles of the cones,
in comparison with the jets, can also be explained by a simple wide
ionised outflow in which the radio jet is simply a central high
velocity component
(Wilson et al. 1993). In Circinus the radio jets close to the nucleus have been
inferred from the
observations of Davies et al. (1998)(47)
and the ionisation cone(48)
in the form of a
unipolar (to the north-west only) V-shaped outflow,
Fig. 9. The highly ionised state of the highly excited
(Oliva et al. 1994) low density (ne ~ 40 cm-3,
Marconi et al. 1994)
supersonic (Veilleux & Bland-Hawthorn 1997) gas is confirmed by the
presence of the [NeIII,
V, VI], [SIV], [MgV,VII,VIII], [OIV] and [SiIX] species
(Moorwood et al. 1996a). The various outflow features in Circinus are
summarised
in Table. 3, and the results appear to support the
hypothesis that the jet drives the ionisation cone, together with an
envelope of molecular gas, out along the rotation axis of the
molecular ring(49)
(Curran et al. 1998; Curran et al. 1999; Figure 9).

This geometry is also evident in Mrk 231, where the ~ 100 pc scale
gas disk appears to be centred on the AGN and perpendicular to the
radio lobe axis
(Carilli, Wrobel & Ulvestad 1998), Fig. 10.
So like Circinus, this suggests a continuous alignment of the
disk/ring/torus. This scenario, however, may be exceptional according to the
results of Schmitt & Kinney (1996); Schmitt et al. (1997) who find that,
although the larger
scale molecular ring is expected to be coplanar with the disk of the
galaxy
(McLeod & Rieke 1995), it will not in general be coplanar with the obscuring
torus. Although, admittedly from a very simple model, Curran (2000) finds that
there is tendency for the torus, ring and large scale disk to be
approximately aligned (within
30°). In fact the situation
may be somewhat more complicated than this, as Maiolino & Rieke (1995) suggest
that the molecular ring will cause some obscuration of both the narrow
and broad line regions. This will have the consequence of dimming the
luminosity of the narrow lines, which are more extended in Sy2s
(Schmitt & Kinney 1996), above inclinations of
50°, thus making these
galaxies appear as type 1.8 and 1.9 Seyferts. This idea is somewhat supported
by
Schmitt & Kinney (1996) who find that galaxies of inclinations as high
as 60° will
still appear as Sy1s, although this may merely indicate the large
opening angle of the torus (CAion in
Table 2). Also,
Kohno et al. (1996) propose that the large scale (~ 100 pc) molecular
disk contributes significantly to the obscuration of X-rays from the
weak Seyfert nucleus in M51.

Figure 10. A schematic model
of the Sy1 Mrk 231 (adapted from
Carilli, Wrobel & Ulvestad 1998) showing the orientation of the gas
disk components with
respect to the radio lobes, which are shown by the centre contours. The
gas disk is also known to have such a structure in Circinus
(Elmouttie et al. 1998a; Curran et al. 1998; Elmouttie et al. 1998b).

Returning to the molecular outflows, although it is as yet
uncertain whether they are common to all Seyferts,
Irwin & Sofue (1992) derive the presence of a such an outflow directed along
the VLBI jet in the Sy2NGC 3079(50),
and their presence cannot be ruled out in other
galaxies (Curran 2000)
(51).
A molecular
outflow has also been observed in the previously mentioned star-burst
galaxy M82
(Nakai et al. 1987) and, like the outflow inCircinus(52),
this extends to
500 pc
in a direction normal to the galactic disk at a velocity of
~ 200 km s-1. Unlike Circinus, however, this outflow takes the form
of hollow cylinder rather than a cone surrounding the ionised gas,
thus indicating the absence of the compact dense obscuring torus
which would collimate the supernova driven outflow into a conical
shape (Chevalier & Clegg).

(42) The cone is often referred to as
the extended narrow line region, although the mechanism behind
the narrow line region (Section 1) may not be
the same (Morris et al. 1985; Unger et al. 1987; Storchi-Bergmann,
Wilson & Baldwin 1992; Hjelm & Lindblad 1996).
Back.
(43) If this were directed along the axis
of the galaxy (Wilson 1991), it would
place the axis of the outflow perpendicular to the large scale disk.
Back.
(44) Note that the opening angles of the
ionisation cones suggests that the obscuration allows only
1/5 of the sky to be visible
from the central
engine. This is consistent with the fact that there appears to be only
one Sy1 for approximately every five Sy2s (Maiolino & Rieke 1995). Note
also that
the large-scale radio structures in Seyfert galaxies may be weaker
versions of those observed in radio-loud AGNs and that pc-scale jets
aligned with these have been observed (Murray et al. 1999; Kukula et
al. 1999), although
ionisation cones have yet to be detected in radio-loud sources (e.g.
Peterson 1997). Back.
(45) Research Papers B and
F.
Back.
(46) Pedlar et al. (1998) propose a
model where the torus
is a consequence of the weak radiation emitting from the equator of
the continuum source, whereas the cone arises from gas ionised by the
strong polar radiation. Back.
(47) Elmouttie et al. (1995) also observe large
scale radio lobes. Back.
(48) In Circinus
the IR luminosity from the ionising radiation is greater than that
from the star-burst (Moorwood et al. 1996a; Siebenmorgen et al. 1997).
Back.
(49) This indicates that the ring
axis is coincident with that of the obscuration collimating the
outflow. Back.
(50) Like Circinus, their CO 1 -> 0
observations also show the presence of a 750 pc molecular ring, see
Table 1.
Back.
(51) Quillen et al. (1999)
have recently observed
excited molecular hydrogen (
= 1.9750 µm) coincident with the
ionised gas in NGCs 2110, 5643 and Mkn 1066 and we plan to search for
molecular outflows in other Seyfert galaxies, Chapter 3/Appendix D.
Back.
(52) A molecular ring
is also present in M82, Section 2.2.
Back.